The present invention relates to bore hole tools, and more specifically to a core sample orientation tool used for taking core impressions in bore holes.
Core drilling is employed in various fields and particularly in the mining and petroleum industries in order to secure samples of the earth's crust at predetermined depths to secure subsurface geological data for the purpose of analysis and study. Full knowledge of the characteristics of a subsurface formation can be obtained only if the precise location and orientation of the core sample, before it was taken, can be determined. Core orientation is accomplished by removing a sample of the core from a bore hole and orienting it in space in precisely the same position that it occupied in the bore hole. Normally such orientation of core samples is done for the purpose of aiding in the proper evaluation of subsurface geology and thus determining the proper drilling program for the bore hole or well. A further purpose is to facilitate the proper development of an oil field by ascertaining the dip and strike of subsurface formations, such data being of considerable importance to the geologist. The dip of a stratum is the largest angle between the plane of the stratum and a horizontal plane. The strike of the stratum is the direction that the intersection of the plane of the stratum and a horizontal plane makes with respect to north.
Although data for determining dip and strike can be obtained by other methods, as for example, a comparison of several bore holes by electric logging and bore hole profile logging techniques, such methods rely on the difference in properties between the successive strata and are sometimes not applicable where massive bodies of rock are encountered. Hence, there is a need for orienting cores to obtain the fullest information when boring a bore hole or well.
During the drilling of a bore hole in the search for oil or minerals, core samples are cut from the formations being traversed and are removed to the earth's surface for examination. Various important information can be obtained from such a core. For example, if any bedding planes are observable in the core, the strike and dip of these planes (and hence of the formation from which the core was obtained) can be determined. The true directions of strike and dip, however, can be determined only if the core can be oriented (in space) in the same way that it was oriented in its original place in the formation.
In order to ascertain the dip and strike of strata existing underground, it is first of all necessary to sink a bore hole to intersect the stratum to be investigated and to extract a piece of bore core from the lower end of the bore hole. A bore hole of any considerable depth, however, always deviates from its original vertical direction so that readings taken from the piece of bore core when mounted in a vertical position will not correctly represent the dip and strike of the selected stratum.
Geologists have long recognized the value of oriented cores. The initial and most obvious use of oriented cores is to determine the dip and strike of inclined strata which permits a more complete interpretation of structural complications. Secondary recovery programs such as water flooding have also shown that it is highly desirable to know the extent and direction of any preferential permeability which formations may exhibit. The great mass of subsurface geological data obtained over the past several years in the concerted geologic research efforts of most oil companies have amplified the need for oriented subsurface data of every type. This potential is not restricted to the petroleum industry, but is equally applicable wherever drilling cores are obtained, such as in the exploration for development of uranium, metallics, non-metallics and other minerals, the engineering and construction of dam sites, tunnels, bridges and the like, quarrying operations and many others. However, this data is of use only when drilled cores are oriented with the utmost accuracy and then only when the orientation can be accomplished economically. Many devices, methods and techniques have been employed for accomplishing core orientation. Some are complicated and time-consuming, some are of limited accuracy, and others have not been entirely satisfactory for various reasons.
One known technique employs a magnetic needle which is held fixed in the position of core orientation. Another technique scribes a mark on the core itself. Another technique includes a marker driven into a hole in the core which remains there while the core is being drilled out and brought up to the surface. Another technique injects a charge of magnetically susceptible particles adjacent to the rock to be sampled. Another technique employs a luminous ball and a light-sensative surface. Another technique subjects the rock to be sampled to a strong magnetic field of known orientation before the core is taken so that after cutting, the original orientation of the core may be ascertained by making use of the remanent induced magnetism. Another technique compares the physical property of a side wall core of known orientation with a conventional core. Still another technique uses a pin carrier ring and movable ball to determine the orientation of the core at the bottom of a bore hole. Another technique uses an orientation indicating instrument which secures the core stub thereto.
It is also known to employ a pendulum which may oscillate in all directions relative to the middle axis of the core tube and having a borer at its end to directly mark the face of a core to determine the angle of inclination of dip, see U.S. Pat. No. 2,011,979 (Martienssen). This device suffers from the same problems as the other aforementioned devices that directly mark the core, i.e., debris, water or mud in the hole which may prevent suitable marking and a separate hole survey is needed to orient the core for each core sample.
Still another technique which is used by the Bureau of Reclamation employs a putty receptacle attached to aluminum or plastic manual setting tools. Once a drill hole has been started, a mark is made at twelve o'clock on the hole collar. Next, the putty receptacle is pushed into the hole with the setting tools, maintaining constant alignment between a scribe line on the tools and the mark on the hole collar. When hole bottom is reached, the putty is pressed against the bottom and an impression is made. The setting tools and putty impression are then removed from the hole. The putty receptacle is then placed in a tray in the configuration where the scribe line is in its uppermost position (twelve o'clock). When the next length of core is removed from the hole, it is placed in the tray and matched to the impression in the putty. The drill core can now be mapped for fracture orientations and spacings. This technique requires precise matching of the scribe lines by the operator and is limited to manual application in shallow holes, i.e., approximately 200 feet.